1. Field of the Invention
The present invention relates to an elastic-wave monitoring device which monitors the distribution of an elastic wave by performing measurement using a laser light, and to a surface-acoustic-wave device for use with the elastic-wave monitoring device.
2. Description of the Related Art
SAW (surface acoustic wave) devices and related components which utilize an elastic wave, such as a surface acoustic wave, have been briskly developed. In order to study the behavior of the SAW devices and use it for improvement of the SAW devices, there is an increasing demand for the technique to observe the elastic wave.
For example, there is known the technique which uses the interferometer to observe the distribution of a surface acoustic wave in a SAW device.
By this technique, the surface displacement of the surface acoustic wave is measured from a small change of optical intensity of a coupled light beam, which is obtained by using interference of a light beam reflected by the surface of the measured object and a light beam reflected by the reference mirror. Based on the principle that the displacement in the direction perpendicular to the surface changes the optical path difference, the surface displacement of the surface-acoustic-wave in the direction perpendicular to the surface is measured by this technique.
On the other hand, a conceivable method to observe the on-surface displacement of the measured object is to monitor the elastic-optical effects (stress birefringence) produced by distortion of the surface acoustic wave, by detecting a change of a polarization state of a transmission light through the measured object.
A polarization microscope is used to observe the stress distortion in the measured object from the double refraction by the elastic-optical effects. In addition, by using the composition that is the same as the optical modulator using the electro-optic effects, it is possible to detect the double refraction from the optical intensity change conversely.
In addition, Japanese Laid-Open Patent Application No. 5-503862 discloses an ultrasonic sensor which receives the incoming circularly polarized light at the polarization-holding-type fiber and outputs the degree of mode coupling excited by the incident supersonic wave.
By the method which performs the measurement using the interferometer, only the perpendicular displacement of the surface acoustic wave in the direction perpendicular to the surface of the measured object can be measured.
However, the surface acoustic wave used by the SAW device is composed of not only the perpendicular displacement but also the on-surface displacement. Actually, the SAW device using a 36-degree Y cut plate of lithium tantalate (LiTaO3) crystal shows almost only the on-surface displacement. Otherwise, it uses the surface acoustic wave in which the overall energy is created by the on-surface displacement. Therefore, the measurement method using the interferometer is not suitable for observing the distribution of the surface acoustic wave in such a SAW device.
When considering the detection of the elastic-optical effects (stress birefringence) from a change of a polarization state of transmission light in order to observe the on-surface displacement, the crystal substrate of the SAW device has the optical anisotropy and the surface of the crystal substrate is a cut plane having a complicated angle. With a simple composition like the optical modulator utilizing the electro-optic effects, it is difficult to perform the observation of the surface acoustic wave in the actual SAW device.
FIG. 1 shows a conventional elastic-wave monitoring device. In FIG. 1, only the composition of the optical system of the conventional elastic-wave monitoring device is shown. The conventional elastic-wave monitoring device monitors refractive index change produced by the elastic-optical effects, by detecting a change of a polarization state of a transmission light. The composition of FIG. 1 is used for an optical modulator or the like.
In the optical system shown in FIG. 1, the circularly polarized light 2 from the light source is incident to the crystal 3 that is a measured object. The light 2 passes through the crystal 3, and further passes through the phase-difference compensating plate 4 and the polarizing filter 41, in this order. The resulting light from the polarizing filter 41 is incident to the photodetector 6.
Suppose that the X-axis 14 and the Y-axis 15 are predetermined with respect to the propagation direction of light so that the index of refraction of the crystal 3 is defined. The refractive index of the crystal 3 is composed of the x-axis direction component “nx” and the y-axis direction component “ny”.
FIG. 2A and FIG. 2B are diagrams for explaining the relation of a change of polarization to a change of refractive index in the optical system of FIG. 1.
As shown in FIG. 2A, the circularly polarized light is converted into the elliptically polarized light having the principal axis in the direction of 45 degrees, due to the change Δn of the refractive index in the x-axis direction.
The optical intensity I (indicated by the distance of the line 46 and the origin of the index ellipsoid shown in FIG. 2B) of the 45-degree direction polarization component is represented by the formula:I=Io+CΔn (where Io is the intensity when there is no change of the refractive index, and C is a factor).
In the composition of FIG. 1, the polarizing filter 41 has the transmission axis 42 in the 45-degree direction. The optical intensity of the polarized light from the polarizing filter 41 is indicated by the optical intensity I mentioned above.
As shown in FIG. 2B, the sensitivity of the optical intensity of the polarized light from the polarizing filter 41 is the maximum when the direction of the transmission axis 42 of the polarizing filter 41 is the 45-degree direction between the X axis and the Y axis which are used to define the index of refraction of the crystal 3. In the 0-degree direction, the optical intensity is not sensitive.
As the anisotropic crystal has the index of refraction that varies with the propagation direction of the transmission light, the index of refraction is determined for each of the directions of the principal axes of an ellipse formed by intersections of the index ellipsoid of the crystal and a plane perpendicular to the propagation direction of the transmission light and passing through the origin of the index ellipsoid. The magnitude of the index of refraction is indicated by the length of one of the principal axes of the ellipse.
When the elastic-optical effects occur, the index ellipsoid is deformed and it serves as a change of the index of refraction to the light in an arbitrary propagation direction. The final change of the index of refraction is determined with the distortion component of the surface acoustic wave, the elastic-optical effects and the propagation direction of light. The change, which is detected with the composition of FIG. 1, does not necessarily appear with sufficient convenience.
FIG. 3A and FIG. 3B are diagrams for explaining the relation of a change of polarization to a change of the direction by which a refractive index is defined.
When the crystal 3 as the measured object is provided in the form of a 36-degree Y cut plate of a lithium tantalate crystal, and the light is incident to the cut surface of the crystal 3 at right angles to the cut surface, it is found that the axes by which the index of refraction is defined are rotated as shown in FIG. 3A. However, the value of the index of refraction itself does not substantially change. It is difficult for the conventional elastic-wave monitoring device of FIG. 1 to detect a change of the surface acoustic wave in such a case.
Moreover, although it is possible for the conventional elastic-wave monitoring device of FIG. 1 to observe the distribution of the elastic wave in the SAW device, there is the problem that the behavior of the elastic wave in a non-sensitive range of the SAW device in which the light does not penetrate the SAW device cannot be observed. The non-sensitive range is, for example, the range of the SAW device in which the metal electrode is formed on the crystal substrate.
The metal electrode on the SAW device functions as the mirror to reflect the incident light. Hence, the light incident to the metal-electrode range of the SAW device is reflected, and the metal-electrode range becomes the non-sensitive range in which the incident light does not interact with the crystal substrate of the SAW device.